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Core logging is the systematic recording of what a drill core contains: rock types, alteration, mineralisation, structures and rock strength. A good core log turns a few hundred metres of grey cylinders into data that supports resource estimates, geological models and the next drill hole. A poor log wastes the money spent on drilling. This guide walks junior geologists through core logging in the order the work is normally done, from the moment core arrives at the core shed to the day it goes into storage.
Why Core Logging Matters
Diamond drilling is the most expensive part of most exploration programmes. The core is the permanent physical record of the ground, and the log is how that record is read by everyone who was not in the shed. Investors, competent persons and future geologists will all rely on your descriptions, often years later.
Consistency matters more than detail. Two geologists should describe the same core in the same way, using the same codes. That is why companies write a logging manual before drilling starts.
Step 1: Prepare the Core Shed and Stay Safe
Set up a well-lit, covered area with sturdy logging tables, water, a hand lens, a scratch kit, a magnet, a tape measure, a protractor or core-orientation tool, acid and a marker. Safety comes first. Wear safety boots, gloves, glasses and dust protection when cutting or splitting. Lift core trays with two people, because a full tray is heavy. Keep walkways clear of core boxes and water.
Step 2: Check Core Handling, Orientation and Depth Marks
Lay out the core boxes in order and fit broken pieces back together. Confirm that each box is labelled with hole number, box number and from-to depths. Check the driller’s wooden or plastic blocks, which show the depth at the end of each run, against the metres marked on the core.
If the core was oriented in the hole, a reference line (often the bottom-of-hole line) is drawn along the core. Join the pieces and extend this line across breaks. Where the line cannot be matched, mark the piece as unoriented rather than guessing.
Step 3: Measure Recovery and RQD
Core recovery is the length of core recovered in a drill run divided by the length drilled, expressed as a percentage. Low recovery often points to broken or soft ground, and in mineralised zones it can bias grades.
Rock Quality Designation, or RQD, is the total length of core pieces longer than 100 mm divided by the run length, also as a percentage. Many standard guides define it on core of about 54 mm diameter. It is a simple index of how fractured the rock is, and it feeds into slope and tunnel design later.
Step 4: Photograph the Core, Wet and Dry
Photograph every box before anyone cuts, samples or disturbs it. Include a label board with hole number, box number and depths, plus a colour chart. Use even light and shoot square-on from directly above. Some teams photograph dry core first and then wet it with a spray bottle and photograph again, because wetting brings out colour and texture. Whatever you choose, keep the method the same across the project. Back up the images the same day.
Step 5: Log Lithology, Alteration and Mineralisation
Lithology is the rock type. Record the main rock unit, its colour, grain size, texture and any contacts, along with the angle of each contact to the core axis. Use the project code list, such as a short code for basalt, shale or granite, instead of free text.
Alteration is the chemical change of rock caused by hot fluids, for example silicification, sericite, chlorite or carbonate. Record the type, intensity and style (pervasive or in fractures). Alteration often forms a halo around mineralisation, so it guides where to look next.
Mineralisation is the ore minerals themselves. Note the mineral (pyrite, arsenopyrite, chalcopyrite, visible gold), its percentage, texture and form, such as disseminated grains or veinlets. Estimate percentages with a visual chart to reduce bias.
Step 6: Log Veins and Structures with Alpha and Beta Angles
Record each vein by type (quartz, quartz-carbonate), thickness, texture and any sulphides or gold. Structures include faults, shear zones, joints and foliation.
On oriented core, two angles fix the orientation of a plane. Alpha is the acute angle between the plane and the core axis. Beta is the angle measured around the circumference of the core from the orientation reference line to the plane. Conventions for beta differ between companies, so write yours into the manual. Software can convert alpha and beta, with hole azimuth and dip, into the true dip and dip direction of the plane. These are the measurements used later on stereonets for structural interpretation.
Step 7: Record Weathering and Oxidation Zones
In tropical Africa the top tens of metres of many holes pass through deeply weathered rock. Note the base of soil, saprolite (rotted rock that keeps its original fabric), saprock and fresh rock, and any oxide-to-sulphide transition. Weathering changes metal recovery, density and sample handling, and it can leach or enrich gold. Record the depth to fresh rock in every hole.
Step 8: Add Geotechnical Logging
Geotechnical logging describes how the rock behaves mechanically. Typical entries are recovery, RQD, fracture frequency, joint roughness and infill, and a field estimate of rock strength. This data supports pit slope design and underground planning. Collecting it at the start is cheaper than re-drilling later.
Step 9: Mark Sampling Intervals and Measure Density
Sample intervals follow geology, not a rigid grid. Honour contacts, vein edges and alteration boundaries, and keep lengths within the project limits (often about 0.5 to 1.5 m). Mark the cutting line so the same half is always sampled, and keep the other half in the box.
Bulk density is measured on representative pieces by weighing them in air and in water, often with wax coating for porous or oxidised rock. Density converts volume to tonnes, so it directly affects resource estimates. Measure each major rock type and each weathering zone.
Step 10: Use Hand-Held XRF and Magnetic Susceptibility Carefully
A hand-held XRF (pXRF) gives quick chemical readings, useful for pathfinder elements and lithology. Treat results as indicative only unless calibrated against lab assays, and never use them as the grade of record. A magnetic susceptibility meter measures how strongly rock responds to a magnetic field. It helps to separate rock units and to tie core to magnetic survey data. Take readings at regular spacing on clean, dry surfaces.
Step 11: Enter the Data Digitally
Enter data straight into a logging tool on a laptop or tablet, or into a template with drop-down lists. Digital logging software with validation rules prevents overlapping intervals, missing depths and misspelt codes. Export to a central database with separate tables for collar, survey, lithology, alteration, structure, geotechnics and assays. Fix errors the same day while the core is still in front of you.
Step 12: Apply QA/QC to Logging and Sampling
Quality assurance and quality control (QA/QC) applies to logging too. Review a few holes with another geologist to check consistency. In sampling, insert certified reference materials (standards), blanks and duplicates at a regular rate so the laboratory can be checked. Keep chain-of-custody records for every sample from the core shed to the lab.
Step 13: Store the Core
After logging, photographing and sampling, close the boxes, stack them on racks off the ground and record the storage location. Protect core from rain, sun and termites. Keep mineralised intervals and reference sections accessible. Core is evidence, and regulators and buyers may ask to see it.
A Simple Example Log
| From (m) | To (m) | Recovery (%) | RQD (%) | Lithology | Alteration | Mineralisation | Notes |
|---|---|---|---|---|---|---|---|
| 0.0 | 12.0 | 70 | 0 | Soil and saprolite | Clay | None | Oxidised, weathered |
| 12.0 | 38.5 | 96 | 78 | Basalt | Chlorite, weak carbonate | Trace pyrite | Fresh rock from 14 m |
| 38.5 | 41.0 | 98 | 85 | Quartz vein in sheared basalt | Strong silica, sericite | 2% pyrite, 1% arsenopyrite | Alpha 40, beta 110; sample 3 intervals |
| 41.0 | 60.0 | 99 | 90 | Basalt | Chlorite | Trace pyrite | Fresh, massive |
This is a made-up teaching example, not real data.
Conclusion
Good core logging is careful, consistent and safe. Orient and label the core, record recovery and RQD, photograph before sampling, log what you see with agreed codes, measure density, and keep your data clean and backed up. Follow the same steps on every hole and your logs will serve the project for its whole life.
